This invention relates to a method of preparing magnetic toner particles by dispersion polymerization methods and more paticularly to a method of preparing toner particles having a concentration of magnetic material greater than 45 percent by weight by dispersion polymerization methods.
Magnetic toner particles have recently found application in the development of electrostatographic images as single component developers. Single component magnetic developers are particularly applicable in small copiers where size constraints are an important aspect. In some applications, the toner particles are field dependent, that is they are capable of acting as conductors under high electrical fields and capable of acting as insulators under lower electric fields. This permits the development of the latent electrostatic image by induction and the transfer to plain paper by electrostatic techniques.
It has been learned through experimentation that the toner particles should have a concentration of magnetic particles of from about 50 to about 60 percent by weight. Toner particles with such high magnetic particle concentration can be obtained by bulk methods and by spray drying methods. In the bulk method of preparation the magnetic particles and the polymer are blended together by any suitable technique including, for example, mastication on a rubber mill, in Banbury mixers, and the like with subsequent particle size reduction of the bulk material to a suitable size for use as toner. One disadvantage of this technique is that the particle size of the toner is not uniform, thus, classification methods are required in order to achieve a toner composition of uniform particle size.
Spray drying techniques are conducted by dissolving a suitable polymer in a solvent to which is added the proper quantity of magnetic pigment. Subsequently, toner particles are formed by spray drying the solution to achieve toner size spherical particles. This technique is capable of achieving magnetic toner particles having the suitable concentration of magnetic material contained therein however it is generally a rather expensive process and has not found commercial acceptance because of this reason.
A third method of preparing toner particles has been termed the dispersion polymerization method. In this technique, the magnetic particles are first mixed with monomer together with a reaction initiator and subsequently this mixture is suspended in an aqueous medium in which the reaction takes place within each particle resulting in the formation of toner size particles which are then removed from the aqueous medium by conventional techniques. A problem that exists with this technique is that the maximum concentration of magnetic particles in the completed toner particles is from about 40 to about 45 percent by weight. This is not the optimum percentage for use in single component magnetic developing methods.
It is desirable, however, to utilize dispersion polymerization method for the preparation of single component magnetic developer because of its low cost, narrow resulting toner particle size distribution and the reduced number of processing steps employed.
It would, therefore, be desirable to provide single component magnetic developer particles containing greater than 45 percent magnetic particles by weight by a dispersion polymerization method in order that the inherent advantages of the dispersion polymerization method can be obtained.